Fix Firefox to be more reliable on versions over 45

* Firefox behaves differently between an ActionChains click and a "real"
  click. The ActionChains click does not try to wait for page navigation
  as such, navigations often fail because the page is changed from under
  it.
* We now use a "real" click if a navigation has been requested. This now
  also initiates a sleep for 1s immediately after we instruct marionette
  to click. This is then enough time for any CSS animations to finish up
  before we continue running any more selenium methods to check for page
  objects to be displayed.

src/widgetastic/browser.py

@@ -354,7 +354,11 @@ def click(self, locator, *args, **kwargs):
         el = self.move_to_element(locator, *args, **kwargs)
         self.plugin.before_click(el)
         # and then click on current mouse position
-        self.perform_click()
+        if nav_click and self.browser_type.lower() == 'firefox' and self.browser_version > 52:
+            el.click()
+            time.sleep(1)
+        else:
+            self.perform_click()
         if not ignore_ajax:
             try:
                 self.plugin.ensure_page_safe()